1. Field of the Invention
The invention relates to a technical field of a crystal device for surface mounting which is appropriate for miniaturization, and in particular, to a crystal device hermetically encapsulating a crystal element therein by seam welding.
2. Description of the Related Art
A crystal device for surface mounting such as a crystal unit, a crystal oscillator, or a crystal filter is known as a frequency control element. For example, a crystal unit for surface mounting (hereinafter, referred to as a surface-mounted unit) is integrated into an oscillator circuit, to be built into various types of electronics as a source of frequency or as a time reference. In recent years, the miniaturization thereof has been further advanced, which has brought about their planar outlines which are 2.0×1.6 mm or less, for example.
FIGS. 6A to 6C are diagrams for explanation of one example of a related art surface-mounted unit. FIG. 6A is a cross-sectional view of the related art surface-mounted unit, FIG. 6B is a plan view of the related art surface-mounted unit without a metal cover, and FIG. 6C is a plan view of the related art surface-mounted unit to which the metal cover is jointed.
The related art surface-mounted unit 200 is configured such that a crystal element 2 is housed in a ceramic case 1 in a rectangular form whose cross section is formed to be concave, and a metal cover 3 is jointed by seam welding to hermetically encapsulate the crystal element 2. The ceramic case 1 is composed of a bottom wall 1a and a frame wall 1b, and a metal ring 4 is fixed to the upper surface of the frame wall 1b serving as an end face of the opening by, for example, silver solder (not shown) (see, for example, JP-A-2007-173976)
In this case, the outline of the metal ring 4 is made smaller than the outline of the ceramic case 1, to prevent the metal ring 4 from protruding from the end face of the opening of the ceramic case 1. Then, the outer circumference four corners of the metal ring 4 are formed into arc forms (curved portions) with a radius of curvature r1, for example. The outer circumference four corners of the metal ring 4 are provided so as to avoid through holes for dividing a ceramic sheet provided at outer circumference four corners of the ceramic case 1. The inner circumference of the metal ring 4 is matched to the inner circumference of the frame wall 1b of the ceramic case 1.
Then, external terminals 5 are provided to the four corners of the outer bottom face of the ceramic case 1 (bottom wall 1a) and crystal holding terminals 6 are provided to the both sides of one end of the inner bottom face. A pair of the external terminals 5 obliquely facing each other at the four corners of 5 the outer bottom face is electrically connected to the crystal holding terminals 6 on the inner bottom face through a lamination plane and unillustrated through holes (electrode through holes). The other pair of the external terminals 5 obliquely facing each other is connected to the metal ring 4 through electrode through holes and the like.
The crystal element 2 has excitation electrodes 7 on both principal surfaces thereof and leading electrodes 8 extended from the excitation electrodes 7 on the both sides of one end thereof. The extended both sides of the one end of the leading electrodes 8 are fixed to the crystal holding terminals 6 by an electrically conductive adhesive 9, and the both are electrically and mechanically connected.
The metal cover 3 is formed into a similar shape to the outline of the metal ring 4, for example, and has arc-formed curved portions with a radius of curvature r2 (=r1) at its outer circumference four corners. Thereby, a ratio A2/A1 between a length A1 of a long side of the metal ring 4 and a length A2 of a straight portion of along side except for the curved portions of the outer circumference four corners thereof, and a ratio C2/C1 between a length C1 of a long side of the metal cover 3 and a length C2 of a straight portion of the long side except for curved portions of the outer circumference four corners thereof become the same.
Further, a ratio B2/B1 between a length B1 of a short side of the metal ring 4 and a length B2 of a straight portion of the short side except for curved portions of the outer circumference four corners thereof, and a ratio D2/D1 between a length D1 of a short side of the metal cover 3 and a length D2 of a straight portion of the short side except for curved portions of the outer circumference four corners thereof as well become the same. Incidentally, by setting the radius of curvature r2 of the outer circumference four corners of the metal cover 3 equal to the radius of curvature r1 of the outer circumference four corners of the metal cover 4, even when the metal cover 3 is displaced front and back and side to side from the metal ring 4, for example, the outer circumference four corners of the metal cover 3 can be prevented from protruding.
Seam welding energizes between the pair of roller electrodes while making a pair of roller electrodes (not shown) contact the one end side of a set of sides facing each other of the metal cover 3 to press those to rotate so as to travel to the other end side. Thereby, an Ni (nickel) film on the outer circumferential surface of the metal cover 3 is fused by Joule heat so as to be jointed-to the metal ring 4. Then, after the set of the sides facing each other of the metal cover 3 are jointed to the metal ring 4 the other set of sides facing each other are jointed thereto in the same way (see, for example, JP-A-2007-75857).
However, in the related art surface-mounted unit 200 having the above-described configuration, since the arc-formed curved portions preventing the metal cover 3 from protruding from the metal ring 4 are provided to the outer circumference four corners of the metal cover 3, there is a problem that airtight loss is caused depending on welding conditions for performing seam welding of the metal cover 3 onto the metal ring 4.
For example, when a speed of the electrode rollers in seam welding is kept constant, and an electric current value is strictly limited, as shown in FIG. 7A, the Ni film 10 on the surface of the metal cover 3 is sufficiently fused even on its straight portions and curved portions of the outer circumference four corners to be jointed to the metal ring 4. Incidentally, FIG. 7A is a plan view (view of end face of the opening) of the ceramic case 1 from which the metal cover 3 is stripped off after the seam welding.
In this case, the Ni film 10 of the contact face on the outer circumference of the metal cover 3 with respect to the metal ring 4 is fused to be jointed to the metal ring 4. Accordingly, the contact face between the metal ring 4 and the metal cover 3 basically becomes a sealing path. Incidentally, the metal ring 4 is formed by press working, and an inclined surface is formed on its inner circumferential surface, which generates unjointed areas of the inner circumference of the surface of the metal ring 4. The areas are omitted in the drawing.
In contrast thereto, if an allowable range for electric current value is expanded to relax limits thereon, as shown in FIG. 7B which is an enlarged view of a part shown by O-shaped dotted line in FIG. 7A, fusion of the Ni film on the curved portions of the outer circumference four corners is insufficient to cause poor jointing, which shortens a sealing path L. Then, if the sealing path is shortened, for example, its joint strength as well is reduced, which causes airtight loss by impact or the like.
In this case, since the electrode rollers in seam welding travel on straight lines along one set and the other set of respective sides facing each other, the contact between the electrode rollers and the metal cover 3 in the curved portions of the outer circumference four corners may be lost. The principal cause for this is because an electric current is not sufficiently supplied by the contact lost. Then, there is a current fluctuation or the like and a change in speed of the electrode rollers in reality even if an electric current value is strictly limited. Thus, poor jointing may be increased, and productivity of the surface-mounted unit may be degraded.
These problems become particularly prominent because the smaller the surface-mounted unit is made, for example, as the planar outline thereof comes to be 2.0×1.6 mm or less described above, the narrower the width of the metal ring 4 is made, which shortens a sealing path-L (a joint width with the metal cover 3) to limit its welding conditions.